1. Field of the Invention
The present invention relates to an optical writing device employed in an image forming apparatus, such as a copier, a printer, a facsimile, etc.
2. Discussion of the Background Art
Recently, an image forming apparatus is demanded to be compact and light at low cost. In particular, since having many parts, a color image forming apparatus is significantly large in comparison with a conventional monochrome image forming apparatus, thereby being highly expected to be compact.
An optical writing device (i.e., an optical scanning device) employed in a commercially available image forming apparatus emits plural light beams to a scan receiving surfaces on image carriers by folding plural times using plural reflection mirrors. However, since using the plural mirrors, the optical writing device tends to be thick as shown in FIG. 12.
As shown, a light beam emitted from a light source, not shown, is finally led to a photoconductive member 101 as a scanning objective via plural mirrors 59 after being deflected by a polygon mirror 54. Since the mirrors 59 need to be arranged not to interfere with each other, a large space is needed, and accordingly, the optical writing device necessarily becomes taller.
The Japanese Patent Application Laid Open No. 58-79215 describes a laser printer use optical system enabling twin color printing in black and red. The Japanese Patent Application Laid Open No. 9-127444 describes a multi beam scanning apparatus capable of increasing accuracy of color supervision while improving workability of maintenance.
The Japanese Patent Application Laid Open No. 10-3048 describes a multi color image forming apparatus for scanning plural photoconductive members using two light sources of a different deflecting direction or a different wavelength. The Japanese Patent Application Laid Open No. 10-282440 describes an optical scanning apparatus and an electro-photographic recording apparatus each employs a multi chip semiconductor laser light source including plural semiconductor laser chips arranged side by side having a different light generation wavelength and distributes and emits laser beams to plural photoconductive member drums.
The Japanese Patent No. 3084829 describes an image forming apparatus for forming an image by deflecting and separating plural laser beams and then emitting those to different sections.
However, since the combined light beam is separated by the separating device in the above-mentioned prior arts, a length of a light path up to the photoconductive member is different from others, or a polygon mirror scanning plane is not in parallel to that extending over light emission receiving positions of the photoconductive members when the light path length is adjusted, as shown in the Japanese Patent Application Laid Open Nos. 58-79215, and 9-127444 as well as the Japanese Patent No. 3084829. Further, an entrance angle of a light beam into the photoconductive member is different from others.
When the plane connecting a polygon mirror scan surface with a light emission position on the photoconductive member is not in parallel to others, the image forming apparatus cannot be downsized admitting that only the optical writing device is downsized due to employment of the plural mirrors. Further, when optical elements for forming a four-color image are substantially symmetrically arranged regarding a polygon mirror serving as a center, the photoconductive members are hardly aligned on a straight line, and are difficult to be applied to a tandem type color image forming apparatus currently spreading widely.
Further, since the optical path length up to the photoconductive member and the entrance angle of the light beam into the photoconductive member are different from others as in the Japanese Patent Application Laid Open Nos. 10-3048 and 10-282440, a diameter of a light beam becomes different on a scan receiving surface of the photoconductive member from others and causes some disadvantages to multi color superimposition on the photoconductive member.